1. Field of the Invention
The present invention relates to a sheet conveying apparatus and an image forming apparatus, particularly to the sheet conveying apparatus and image forming apparatus including a registration roller that corrects sheet skew feeding.
2. Description of Related Art
Conventionally, in the image forming apparatus that utilizes an electrophotographic system to form the image in the sheet such as a recording sheet, the image is obtained by transferring a toner image (visible image) borne on a photosensitive drum or a transfer body to the sheet such as plain paper conveyed from a sheet cassette. The sheet to which the toner image is transferred from the photosensitive drum or transfer body is conveyed to a fixing device to fix the toner image onto the sheet, and the sheet is discharged.
In cases where the images are formed in both surfaces of the sheet, when the sheet in which the toner image is fixed to one of the surfaces passes through the fixing device, the sheet is tentatively guided to a reverse path, and a reverse roller is reversely rotated to convey the sheet to a both-sided conveying path while a rear end of the sheet is set in the head. Then the toner image is transferred and fixed to the sheet reversed by passing through the both-sided conveying path, and the sheet is discharged.
In the conventional image forming apparatus, sometimes a sheet leading end direction is slightly rotated with respect to the sheet conveying direction, that is, sometimes skew feeding of the sheet is generated during a sheet feeding operation for feeding the sheet from the sheet cassette or during sheet conveyance subsequent to the sheet feeding operation. When the skew feeding of the sheet is generated, the image formed on the sheet through the transfer and fixing processes is inclined with respect to the sheet.
Therefore, in the conventional image forming apparatus, a skew feeding correction portion is provided in the sheet conveying apparatus that conveys the sheet to an image forming portion, and the skew feeding correction portion corrects the skew feeding of the sheet to improve accuracy of image forming position.
For example, in the skew feeding correction portion, the sheet conveyed by pre-registration rollers (pair) is abutted on a nip of a stopped registration roller to form a loop (deflection), thereby a sheet leading end is aligned with a direction orthogonal to the sheet conveying direction. After the sheet leading end is aligned, the registration roller and the pre-registration roller are driven to deliver the sheet to the image forming portion.
In the conventional image forming apparatus including the skew feeding correction portion, for example, in order to enhance productivity, the pre-registration roller conveys the sheet at a speed higher than a process speed to abut the sheet on the stopped registration roller. After the sheet is abutted on the registration roller to correct the skew feeding, the pre-registration roller is tentatively stopped, and the registration roller and the pre-registration roller are simultaneously driven in synchronization with image timing (see Japanese Patent Application Laid-Open No. 62-244846).
However, with such a configuration, a large hitting sound (collision noise) is generated because the sheet is abutted on the registration roller at high speed, and the hitting sound becomes larger when the sheet conveying speed is further enhanced with recent increasing process speed. When the sheet is abutted on the registration roller at high speed, sometimes the sheet leading end penetrates through registration roller. In such cases, disadvantageously the skew feeding is not corrected or a leading end margin is deviated.
In order to solve the problem, after the sheet is tentatively stopped near the registration roller, only the roller on the upstream is previously driven to abut the sheet on the registration roller at low speed, thereby reducing the hitting sound (see Japanese Patent Application Laid-Open No. 6-127753).
Similarly, after the sheet is tentatively stopped near the registration roller, only the roller on the upstream is previously driven at the process speed, and the registration roller is driven at the process speed, thereby reducing the hitting sound (see Japanese Patent Application Laid-Open No. 10-152244).
However, in the conventional speed-enhanced sheet conveying apparatus and the image forming apparatus provided therewith, in order to enhance the productivity, it is necessary to convey the sheet at a speed faster than the process speed even after the skew feeding is corrected.
Therefore, in the conventional high-speed image forming apparatus, when the skew feeding of the sheet is corrected, a registration roller drive motor (or clutch) that drives the registration roller is tentatively stopped after a registration sensor detects the sheet leading end as illustrated in FIG. 6A. A relay roller drive motor (or clutch) that drives a relay roller located on the upstream of the registration roller is also stopped.
After the rollers are stopped at the same time as the registration sensor that detects the sheet leading end is turned on, the drive of the relay roller is started in predetermined timing faster than a registration roller re-driving signal by a loop forming time, and the relay roller is driven at the process speed, thereby forming the loop. Then the registration roller is driven at the process speed.
For example, as illustrated in FIG. 6B, the high-speed relay roller located on the upstream of the registration roller is tentatively stopped after the registration sensor is turned on by detecting the sheet leading end. The relay roller is driven to form the loop when a predetermined time TA elapses, and the registration roller is driven when a predetermined time TB elapses.
However, in the case of the drive control illustrated in FIGS. 6A and 6B, until the registration roller reaches the process speed after the loop is formed, the excessive loop is formed because the relay roller is driven at the process speed. That is, when the registration roller is started up after the relay roller is previously driven to form the loop, the excessive loop is formed until the registration roller reaches the constant speed, which results in buckling. Particularly the loop amount becomes larger for a short time as the process speed is enhanced due to the speed enhancement of the image forming apparatus. When the large loop space is ensured, the apparatus is enlarged.
When drive coupling is performed with a clutch in order to shorten a time the registration roller reaches the constant speed, because the clutch has a variation in coupling time of about 10 ms, unevenness of the leading end margin is generated depending on the apparatus. The unevenness of the leading end margin becomes severer in the recent high-speed image forming apparatus. For example, in the image forming apparatus having the process speed of 300 mm/s, unevenness of the leading end margin of 3 mm is generated when the coupling time is varied by 10 ms. It is necessary to provide another control unit in order to correct the unevenness of the leading end margin.
In the high-speed image forming apparatus, because the desired loop amount is formed while the pre-registration roller is started up and accelerated, the loop is eliminated between the registration roller and the pre-registration roller when only the registration roller is coupled with the clutch. As a result, step-out of the motor or a fluctuation in sheet conveying speed, caused by tension between the registration roller and the pre-registration roller, is generated to deviate the leading end margin.
The problem is not generated when the whole drive coupling on the upstream of the registration roller is performed with the clutch. However, in such cases, because a clutch coupling sound is simultaneously generated, another noise is generated. Additionally cost is also increased.
In view of the foregoing, an object of the invention is to provide a sheet conveying apparatus and an image forming apparatus, in which the skew feeding of the sheet can securely be corrected while the noise is reduced.